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Saturn’s largest moon was once a titanic snowball

Titan is already a frigid moon made mostly of ice. But methane gas in its atmosphere keeps the surface just warm enough for a scattering of lakes filled with liquid hydrocarbons.

Scientists have puzzled over Titan’s atmospheric methane because the molecule is easily broken down by sunlight. Calculations suggest that all the methane Titan seems to possess should have been used up within tens of millions of years – a blip in the moon’s roughly 4-billion-year lifetime.

Adding to the mystery, the methane breakdown creates other compounds that rain over the surface, helping to fill the lakes. If used-up methane was replaced, this process would happen constantly, so Titan should be covered not by lakes, but by a global ocean hundreds of metres deep.

A similar event could have taken place on Titan, says Wong. Methane levels may rise and fall if the gas is periodically released from inside the moon. If at some point the methane dropped by a factor of 100, temperatures would fall, and surface liquids would freeze over. A different mix of compounds would also be produced in the atmosphere. So this cold snap would mean the moon’s surface should host lots of compounds called nitriles, which would be solid rather than creating an ocean.

“If Titan’s methane levels dropped by a factor of 100, temperatures would fall and its lakes would freeze”

The New Horizons mission to Pluto could offer early clues, says Wong&colon; “Like Titan, Pluto has an atmosphere that is mostly nitrogen with some methane.” Pluto’s atmosphere is much thinner and colder, but the physics are similar enough that examining its composition could boost the snowball model.

This article appeared in print under the headline “Snowball Titan solves methane mystery?”